Field of the Invention
The present invention is directed to scanning probe microscopes (SPMs), including atomic force microscopes (AFMs), and more particularly, to a method of compensating for deflection artifacts caused by, for example, thermal induced bending of the probe during AFM operation.
Description of Related Art
Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically employ a probe having a tip and which cause the tip to interact with the surface of a sample with low forces to characterize the surface down to atomic dimensions. Generally, the probe is introduced to a surface of a sample to detect changes in the characteristics of a sample. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated.
A typical AFM system is shown schematically in FIG. 1. An AFM 10 employs a probe device 12 including a probe 17 having a cantilever 15. A scanner 24 generates relative motion between the probe 17 and a sample 22 while the probe-sample interaction is measured. In this way, images or other measurements of the sample can be obtained. Scanner 24 is typically comprised of one or more actuators that usually generate motion in three mutually orthogonal directions (XYZ). Often, scanner 24 is a single integrated unit that includes one or more actuators to move either the sample or the probe in all three axes, for example, a piezoelectric tube actuator. Alternatively, the scanner may be a conceptual or physical combination of multiple separate actuators. Some AFMs separate the scanner into multiple components, for example an XY actuator that moves the sample and a separate Z-actuator that moves the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other property of the sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,266,801; and Elings et al. U.S. Pat. No. 5,412,980.
Notably, scanner 24 often comprises a piezoelectric stack (often referred to herein as a “piezo stack”) or piezoelectric tube that is used to generate relative motion between the measuring probe and the sample surface. A piezo stack is a device that moves in one or more directions based on voltages applied to electrodes disposed on the stack. Piezo stacks are often used in combination with mechanical flexures that serve to guide, constrain, and/or amplify the motion of the piezo stacks. Additionally, flexures are used to increase the stiffness of actuator in one or more axis, as described in application Ser. No. 11/687,304, filed Mar. 16, 2007, entitled “Fast-Scanning SPM Scanner and Method of Operating Same.” Actuators may be coupled to the probe, the sample, or both. Most typically, an actuator assembly is provided in the form of an XY-actuator that drives the probe or sample in a horizontal, or XY-plane and a Z-actuator that moves the probe or sample in a vertical or Z-direction.
In a common configuration, probe 17 is often coupled to an oscillating actuator or drive 16 that is used to drive probe 17 to oscillate at or near a resonant frequency of cantilever 15. Alternative arrangements measure the deflection, torsion, or other characteristic of cantilever 15. Probe 17 is often a microfabricated cantilever with an integrated tip 17.
Commonly, an electronic signal is applied from an AC signal source 18 under control of an SPM controller 20 to cause actuator 16 (or alternatively scanner 24) to drive the probe 17 to oscillate. The probe-sample interaction is typically controlled via feedback by controller 20. Notably, the actuator 16 may be coupled to the scanner 24 and probe 17 but may be formed integrally with the cantilever 15 of probe 17 as part of a self-actuated cantilever/probe.
Often, a selected probe 17 is oscillated and brought into contact with sample 22 as sample characteristics are monitored by detecting changes in one or more characteristics of the oscillation of probe 17, as described above. In this regard, a deflection detection apparatus 25 is typically employed to direct a beam towards the backside of probe 17, the beam then being reflected towards a detector 26, such as a four quadrant photodetector. The deflection detector is often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, but may be some other deflection detector such as strain gauges, capacitance sensors, etc. The sensing light source of apparatus 25 is typically a laser, often a visible or infrared laser diode. The sensing light beam can also be generated by other light sources, for example a He—Ne or other laser source, a superluminescent diode (SLD), an LED, an optical fiber, or any other light source that can be focused to a small spot. As the beam translates across detector 26, appropriate signals are processed by a signal processing Block 28 (e.g., to determine the RMS deflection of probe 17). The interaction signal (e.g., deflection) is then transmitted to controller 20, which processes the signals to determine changes in the oscillation of probe 17. In general, controller 20 determines an error at Block 30, then generates control signals (e.g., using a PI gain control Block 32) to maintain a relatively constant interaction between the tip and sample (or deflection of the lever 15), typically to maintain a setpoint characteristic of the oscillation of probe 17. The control signals are typically amplified by a high voltage amplifier 34 prior to, for example, driving scanner 24. For example, controller 20 is often used to maintain the oscillation amplitude at a setpoint value, AS, to insure a generally constant force between the tip and sample. Alternatively, a setpoint phase or frequency may be used.
A workstation 40 is also provided, in the controller 20 and/or in a separate controller or system of connected or stand-alone controllers, that receives the collected data from the controller and manipulates the data obtained during scanning to perform data manipulation operating such as point selection, curve fitting, and distance determining operations. The workstation can store the resulting information in memory, use it for additional calculations, and/or display it on a suitable monitor, and/or transmit it to another computer or device by wire or wirelessly. The memory may comprise any computer readable data storage medium, examples including but not limited to a computer RAM, hard disk, network storage, a flash drive, or a CD ROM.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. Operation is accomplished by moving either the sample or the probe assembly up and down relatively perpendicular to the surface of the sample in response to a deflection of the cantilever of the probe assembly as it is scanned across the surface. Scanning typically occurs in an “x-y” plane that is at least generally parallel to the surface of the sample, and the vertical movement occurs in the “z” direction that is perpendicular to the x-y plane. Note that many samples have roughness, curvature and tilt that deviate from a flat plane, hence the use of the term “generally parallel.” In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. In one particularly preferred mode of AFM operation, known as TappingMode™ AFM (TappingMode™ is a trademark of the present assignee), the tip is oscillated at or near a resonant frequency of the associated cantilever of the probe. A feedback loop attempts to keep the amplitude of this oscillation constant to minimize the “tracking force,” i.e., the three resulting from tip/sample interaction. Alternative feedback arrangements keep the phase or oscillation frequency constant. As in contact mode, these feedback signals are then collected, stored and used as data to characterize the sample. Regardless of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research. Note that “SPM” and the acronyms for the specific types of SP Ms may be used herein to refer to either the microscope apparatus or the associated technique, e.g., “atomic force microscopy.”
When making measurements on the sub-nanometer scale, the potential for artifacts in the data is significant, and therefore system set-up and environment often must be taken into account, Again. AFM monitors the physical interaction between its probe and the sample, and thus the mechanical path between the two becomes critical, not only in its set-up but with respect to how this path reacts to its environment One cause of measurement problems is background contributions to the measured probe deflection, i.e., bending or deflection of the probe caused by factors independent of actual probe-sample interaction. Some sources of these background contributions, such as drift and creep, have been studied and solutions have been attempted with varying success. Others are not as well known. For instance, it has been discovered that adverse effects due to the difference in the thermal expansivity of the probe and sample, as well as in homogeneity of the sample itself, can lead to severely compromised AFM data.
Turning to FIGS. 2A-2C, a bimorph AFM probe 42 having a cantilever 44 and a tip 48 is shown schematically. Such probes are typically microfabricated from a wafer with the cantilever being made of silicon (Si), silicon nitride (SiN3), or silicon dioxide (SiO2). Disposed on the backside of cantilever 44 is a metal layer or coating 46 (for example, to create a reflective surface for the optical detection system, etc.). It is the differences in these materials that can cause the probe to bend as a bimorph. In particular, the thermal expansivity of silicon is approximately 3×10−6 K−1 (silicon and silicon nitride), and approximately 1-2×10−5 K−1 for the metal coating 46 (aluminum (Al) is about 2.22×10−5 K−1 and gold (Au) is about 1.42×10−5 K−1). At a certain temperature (e.g., ambient), T0, the probe does not deflect and the artifact theoretically is not present (FIG. 2A). However, at an increase in temperature at a region of probe-sample interaction, T1>T0, probe 42, with its bimorph properties, and in particular cantilever 44, changes its shape and bends downwardly, an amount δ1 as shown in FIG. 2B, for example. At a lower temperature, T2<T0, as illustrated in FIG. 2C, the probe bends or deflects upwardly to change its shape an amount δ2. The deltas, δ1 and δ2, are schematically shown large in FIGS. 2B and 2C for illustration purposes only. This deflection is small, in the range of sub-nanometer to 100 nm, but when resolving features on the sub-nanometer scale, as is the case in AFM, this deflection results in a deflection artifact that has a significant impact on the resultant data. Note that when referencing temperature changes, the thermal properties are dependent on the combined heat generation/absorption properties of the laser and sample.
This thermal bending or deflection artifact is schematically illustrated in FIG. 3 in connection with a probe 50 imaging an inhomogeneous sample. The resultant adverse effect is shown in FIG. 4. A schematic illustration of a cross-section of an exemplary sample 70 shows a first portion 72 and a second portion 74. These two areas 72, 74 of sample 70 comprise different materials which have different properties; in this case, different thermal conductivities (k). Second portion 74 has a higher thermal conductivity than first portion 72, thus causing a difference in the temperature at which the AFM measurement is made during imaging. This, as a result, yields a height artifact in the AFM image; namely, the image height is lower than the true height at about the second portion 74.
More particularly, because the thermal conductivity on a left hand side 72 of sample 70 is lower than on a right hand side 74, a temperature T1 in a region 76 (left) of probe-sample interaction is greater than a temperature T2 in a region 78 (right) of probe-sample interaction. In response, probe 50 will bend up when scanning from left to right in FIG. 3.
This change in deflection of the probe will lead to a thermal probe height change, δ, in the probe-sample separation, and thus a change in the probe height (base of a probe 50 to apex of tip 54). Probe height H2 on the left will be greater than probe height on the right, H2′, even though the sample surface height will not have changed. This, in turn, causes AFM feedback to compensate for the decrease in probe height by sending a control signal to a Z-actuator 80 to, in this case, drive the probe down toward the sample surface. As a result, the AFM measured sample height will be lower than the actual sample height in that region of higher k. Again, this is the above-described thermal bending induced artifact in the acquired AFM data, and is shown schematically as a line of AFM data 82 that results when imaging sample 70 shown in FIG. 3. While sample 70 is generally flat, as it is imaged, the resultant data includes a thermal induced artifact, indicating a lower height at region “A” (corresponding to region 74 of sample 70 having higher conductivity). This artifact renders it difficult to determine true sample topology because the bottom of the flat sample has multiple height values. Clearly, background contributions to probe deflection, including differences in thermal expansivity of probe materials in the presence of temperature changes, and conductivity of different regions of non-homogeneous samples, etc., lead to such unacceptable artifacts. Note that when operating in an intermittent contact mode, such as TappingMode™ or PFT Mode, the position or height of the probe relative to the sample surface discussed herein (including the Z deflection change 6 due to the thermal effect) is roughly the center position of the peak-to-peak oscillation.
An example of temperature change being introduced to an AFM system is illustrated in FIGS. 5A-5C. Initially, with a large probe-separation and at a temperature T0, a probe 150 having a lever 152 and a tip 154 and a metal coating 156 disposed on the lever is shown in FIG. 5A. Probe 150 does not exhibit any bimorph effect at ambient temperature. Then, when preparing for operation and with the probe still a relatively far distance from the sample surface, a laser beam 158 of the optical detection scheme is directed toward the backside of lever 152. This beam acts to heat probe 150 to a temperature, T1>T0. This heating causes a probe bimorph effect, such as that described in connection with FIG. 3, i.e., probe 150 bends downwardly an amount δ1.
During AFM operation, the separation between probe 150 and sample 160 is reduced to cause the two to interact. As the gap between the two is narrowed, the sample surface acts as a heat sink, with the corresponding heat dissipation causing the temperature to decrease, T0<T2<T1, in the region of tip-sample interaction This as a result, causes the probe to bend or deflect oppositely (upwardly) with a corresponding change in thermal deflection from δ1 to δ2, as illustrated in FIG. 5C. This thermal deflection, or background deflection, affects the measured AFM deflection and thus the measured sample height as follows. The detected upward deflection will be interpreted by the feedback loop as a decrease in sample height, when in reality no change in sample height occurred. This causes the feedback loop to narrow the probe-sample separation (via appropriate control signals to the Z actuator) and return the relative oscillating motion between the two to the AFM oscillation setpoint (TappingMode™, PFT Mode). The result is AFM data that includes an artifact showing a sample height lower than the actual or true height.
Turning next to FIGS. 6A and 6B, the above-described thermal bending artifact also impacts mechanical property measurements of sample surfaces when using AFM to generate force curves, for example, as described in U.S. Pat. No. 7,044,007 to Struckmeier et al., owned by the present Assignee. More particularly, when a sample 404 and a probe 400 including a cantilever 401 supporting a tip 402 approach one another, sample 404 may become a heat sink and dissipate beat from probe 400. Notably, probe 400 may also be heated by the laser of the optical deflection detection apparatus (described later herein—again, the combined heat generation/absorption properties of the sample and laser). The temperature of probe 400 will decrease and cantilever 401 will bend gradually upwardly when tip 402 comes close to sample 404. This causes a slope in the observed cantilever deflection 406, and erroneous force data. A solution to this thermal bending to allow the AFM to correct this induced deflection (and yield a more accurate deflection plot 408 [FIG. 6B] without the artifact) was desired.
Overall, an AFM system and method capable of removing deflection artifacts due to probe deflection caused by non-probe-sample interaction from the measured AFM data was desired.